1) Field of the Invention
This invention relates generally to a waveguide circuit for use for optical communication, optical information management and so forth, and more particularly to a structure for and a method of connecting a waveguide and an optical fiber formed on a waveguide substrate to each other, a waveguide substrate for use for such connection and a method of producing the waveguide substrate as well as an optical fiber with a fiber substrate for use for such connection.
2) Description of the Related Art
In recent years, introduction of optical subscriber systems has been and is being promoted, and it is a subject to be solved to minimize and reduce the costs of optical components of optical subscriber systems. Optical parts employing a waveguide are suitable for mass production by batch processing and for minimization and high integration and are considered promising. However, several technical subjects to be solved still remain.
Above all, the technique of connecting a waveguide formed on a waveguide chip (waveguide substrate) to an optical fiber is essential for reduction in cost, and connection means which are stable in property and suitable for mass production and can be supplied at a low cost have been developed actively in recent years.
For example, such means as shown in FIGS. 71(a) and 71(b) or 72(a) and 72(b) is popularly employed as such connection means.
According to the means shown in FIGS. 71(a) and 71(b), in order to connect a waveguide 2 formed on a waveguide chip (waveguide substrate) 1 and an optical fiber 3 to each other, an end face 2a of the waveguide 2 is first formed into a mirror face by a suitable technique such as polishing and then an alignment operation to adjust the position (X-axis direction, Y-axis direction and Z-axis (optic axis) direction) and the angle (.theta., .phi.) of the optical fiber 3 with respect to the waveguide 2 as seen in FIG. 71(a) is performed to establish a particular positional parallel relationship of the optical fiber 3 to the waveguide 2, and then the waveguide 2 (waveguide chip 1) and the optical fiber 3 are coupled directly to each other as shown in FIG. 71(b) by adhesion, fusion connection or some other suitable connection technique.
On the other hand, according to the means shown in FIGS. 72(a) and 72(b), a lens 4 is interposed between a waveguide 2 and an optical fiber 3 as shown in FIG. 72(a), and after the position and the angle of the optical fiber 3 with respect to the waveguide 2 are adjusted similarly as in the means shown in FIGS. 71(a) and 71(b), the waveguide 2 (waveguide chip 1), the lens 4 and the optical fiber 3 are connected and secured to each other as shown in FIG. 72(b) by welding or some other suitable technique. Consequently, light is communicated between the waveguide 2 and the optical fiber 3 by way of the lens 4.
However, both of the connection means described above with reference to FIGS. 71(a), 71(b) and 72(a), 72(b) require adjustment in position at least between the optical fiber 3 and the waveguide 2 (in the means shown in FIGS. 72(a) and 72(b), also the position of the lens 4 must be adjusted), and such adjustment requires a somewhat high skill and much time. Accordingly, reduction in cost by mass production is difficult with any of the connection means.
Different connection means between a waveguide and an optical fiber are proposed, for example, in Japanese Patent Laid-Open Application No. Showa 64-4710, Japanese Patent Laid-Open Application No. Heisei 2-125209 and Japanese Patent Laid-Open Application No. Heisei 5-257019.
According to the waveguide-optical fiber connection method disclosed in Japanese Patent Laid-Open Application No. Showa 64-4710, a waveguide circuit including a ridge type waveguide and several stopper portions is formed on a circuit board. Meanwhile, a connection jig is prepared which is so constructed that a bottom face thereof closely contacts with a concave surface of the waveguide circuit and an outer wall thereof closely contacts with a side wall of at least one of the stopper portions of the waveguide circuit and which has a guide groove formed thereon so as to make the center of the core of the ridge type waveguide and the center of the core of an optical fiber coincide with each other. The connection jig is fixed to the waveguide circuit with the outer wall thereof closely contacted with the side wall of the stopper portion, and an optical fiber is inserted into the guide groove of the thus fixed connection jig, whereafter the optical fiber thus inserted is fixed to the connection jig to connect the waveguide and the optical fiber to each other.
With the waveguide-optical fiber connection method, however, a stopper portion of a complicated configuration must be formed on a substrate, and this makes a factor which makes an obstacle to reduction in cost by mass production. Further, since an optical fiber is not particularly supported, when it is inserted into the guide groove of the connection jig to fix the optical fiber, on the side thereof remote from the connection jig, the optical fiber and the waveguide cannot be fixed with the core of the waveguide and the core of the optical fiber aligned accurately with each other.
meanwhile, according to the waveguide-optical fiber coupling structure disclosed in Japanese Patent Laid-Open Application No. Heisei 2-125209, a waveguide base member has guide grooves formed in a parallel, predetermined spaced relationship from each other therein and has a waveguide formed thereon. The waveguide has an entrance/exit for an optical signal at an end face of the waveguide base member. An optical fiber holding connector member has a guide pin for engaging with the guide groove, and holds an optical fiber positioned such that, when the guide pin is engaged with the guide groove, the optical fiber is optically coupled to the entrance/exit of the waveguide.
With the waveguide-optical fiber coupling structure, however, in addition to the guide groove for fixing an optical fiber, another guide groove is formed in the waveguide base in order to position an optical fiber with respect to the waveguide, and a guide pin for engaging with the additional guide groove must be provided on the optical fiber holding connector member, which makes the waveguide-optical fiber coupling structure complicated.
Further, according to the waveguide apparatus and a process of producing the waveguide apparatus disclosed in Japanese Patent Laid-Open Application No. Heisei 5-257019, a waveguide portion formed on a Si substrate is exposed to an end face of the waveguide apparatus, and a pair of V-shaped grooves each having a V-shaped cross section are formed on the waveguide apparatus. Another pair of V-shaped grooves are formed on a connector, and a pair of guide pins are threaded from the V-shaped grooves of the waveguide apparatus to the V-shaped grooves of the connector so that the waveguide apparatus and the connector are mechanically pressed against each other by way of the guide pins to couple the end face of the waveguide of the waveguide apparatus and the end face of an optical fiber of the connector to each other while aligning the optic axes of the waveguide apparatus and the connector with each other.
Also with the waveguide apparatus and the producing process for the waveguide apparatus, however, in order to position the optical fiber with respect to the waveguide, additional V-shaped grooves must be formed on both of the waveguide apparatus and the connector and guide pins for engaging with the V-shaped grooves must be provided, which makes the waveguide apparatus complicated.
Further, with the connection means disclosed in Japanese Patent Laid-Open Application No. Heisei 2-25209 and Japanese Patent Laid-Open Application No. Heisei 5-257019 mentioned above, the production accuracy of the guide pins and the positioning accuracy between the guide pins and the optical fiber must be high, and it is very difficult to produce the connection portions. Further, as the number of elements increases, the number of steps required for assembly increases, and this makes a factor which makes an obstacle to the mass production and the reduction in cost.
Further, in the techniques described above, in order to position the optical fiber in the direction of its optic axis (Z-axis direction in FIGS. 71(a), 71(b) and 72(a), 72(b)), the optical fiber is moved in a direction of its optic axis until an end face of the optical fiber is abutted with an end face of the waveguide. With the positioning method, however, depending upon the magnitude of the force acting upon the optical fiber to move the optical fiber in the direction of its optic axis, the end face of the optical fiber or the waveguide may be damaged or, as the action of the force continues, the optical fiber may be curved to increase the loss of an optical signal transmitted along the optical fiber. Further, if the optical fiber cannot be moved to an optimum position with respect to the waveguide, then this also makes a factor of increasing the loss of such optical signal.